One or more biometric indicia, such as fingerprints, voiceprints, retinal scans and facial features, are often proposed to be used to identify, or to authenticate the asserted identity of, a user who seeks access to a given resource. Approximately a dozen different biometric indicia have been proposed, but implementation methods for some of these approaches have not been disclosed. Many of these biometric indicia are associated with inherent physiological characteristics of the user's body. Another set of such indicia relate to what may be characterized as neuro-physiological (“N-P”) characteristics that partly reflect a learning or behavioral process and do not rely exclusively on purely physiological features. Use of one or more of these N-P characteristics as a biometric indicium has received relatively little attention, in part because of the perceived difficulty of implementing a procedure to measure such a characteristic. An example is a sequence of bioelectric signals associated with cycles of the heart.
Cardiac muscle is myogenic and is capable of generating an action potential and depolarizing and repolarizing signals from within the muscle itself. An intrinsic conduction system (ICS), a group of specialized cardiac cells, passes an electrical signal throughout the heart. The ICS includes a sino-atrial (SA) node, an atrio-ventrical (AV) node, the bundle of His, right and left bundle branches, and the Purkinje fibers, as illustrated in FIG. 1. These components spread the depolarization waves from the top (atria) of the heart down through the ventricles. The autonomic nervous system modulates the rhythm, rate and strength of cardiac contraction. When the cardiac muscle fibers contract, the volumes within the two atrial or two ventricle chambers are reduced and blood pressure increases. The (smaller) atrial chambers receive blood from the veins and pump the blood into the (larger) ventricle chambers, which pump blood out into the major arteries. The heart cycle normally begins in the right atrial chamber, and spreads to the left atrial chamber and to the two ventricles. The atrial contraction is followed by the ventricular contraction in each cycle.
Simultaneous contraction of the large number of fibers in the ICS generates an electrical field that can be measured at the body surface using an electrocardiograph (ECG). This electrical signal includes a sequence of PQRST complexes, one of which is schematically illustrated in FIG. 2. The time interval between two consecutive R signal peaks, referred to as an R-R interval, corresponds to a heart pulse, with a rate that normally lies in a range of 60-90 beats per minute (bpm). The P signal corresponds to atrial depolarization (right side, followed by left side); the larger QRS complex corresponds to depolarization of the ventricles and (smaller magnitude) repolarization of the atria; and the T signal corresponds to repolarization of the ventricles. A weaker U signal occasionally appears on the chart, representing remnants of ventricular repolarization, but is not shown in FIG. 2.
According to naming conventions accepted by most cardiology workers, a time increment with a straight line amplitude extending between two consecutive signals, for example, from the end of a P wave to the beginning of an immediately following Q wave, is referred to as a “segment;” and a time increment that includes at least one wave, with a graph that is at least partly curved, for example, from the beginning of a Q wave to the beginning of an S wave, is referred to as an “interval.” Herein, a “wave,” such as a P wave, will refer to the curvilinear graph (only) portion of a time interval, not including the associated time segment. Examples of a “wave”, of a “segment,” and of an “interval” are illustrated in FIGS. 3A-3B, 3C and 3D-3G, respectively.
Standard electrocardiography involves multiple recordings of a PQRST complex, referred to as “leads,” which are obtained from a plurality o felectrodes or electrode pairs, placed at spaced apart locations on a patient's body. Unipolar and bipolar leads are frequently used in standard electrocardiography, for the following purposes: (i) standard bipolar limb leads (I, II, III); (ii) augmented unipolar limb leads (aVR, aVL, aVF); and (iii) unipolar chest leads (V1, V2, V3, V4, V5 V6). As illustrated in FIG. 4, the corresponding electrode polarities and locations are set forth in Table I. By convention, the right leg is treated as “ground.”
TABLE ILead Polarities and Locations.LeadNegative Electrode VoltagePositive Electrode VoltageIright armleft armIIright armleft legIIIleft armleft legaVR(left arm + left leg)/2right armaVL(right arm + left leg)/2left armaVF(right arm + left arm)/2left leg
The bipolar lead voltages are recorded with reference to a “ground” electrode located on the right leg. The standard limb leads are configured as an equilateral triangle, referred to as Einthoven's triangle, where the following constraint is imposed: sum of the voltages impressed for the lead pairs I and III is equal to the voltage impressed for the lead pair II. As an example, if the QRS impressed voltages for lead pairs I and III are 0.8 mV and −0.3 mV, the QRS impressed voltage for lead pair II is the algebraic sum, 0.5 mV.
The augmented unipolar lead voltages are recorded between a positive electrode, located on one limb (right arm, left arm or left leg), and two negative electrodes, connected together and located on the other two limbs (left arm/left leg, right arm/left leg and right arm/left arm), respectively.
The chest lead voltages are recorded between a positive electrode located on the patient's chest and a negative electrode represented as a sum of voltages for the three standard limb electrodes (right arm, left arm, left leg). A sum of the three standard limb electrode voltages provides a reference value, sometimes referred to as an “indifferent voltage.” The locations for the six chest leads are well established in the medical field.
FIG. 4 illustrates placement of some of the electrodes used to measure signals and time intervals that are part of an ECG, indicating placement of standard limb electrodes on the right arm RA, on the left arm LA and on the left leg LL. FIG. 5 illustrates use of an Einthoven triangle to estimate an atrial depolarization angle θ(ad;depol) associated with a P wave. One begins with an equilateral triangle EQTR, with right arm (RA), left arm (LA) and left leg (LL) voltages assigned to the three vertices as shown. An augmented voltage aVF is measured, directed perpendicular to a line segment connecting the vertices RA and LA; the vector length of aVF is a deviation of the RA measured voltage from the (expected) median value, (V(RA)+V(LA)/2, described as aVF=V(LL)−{V(RA)+V(LA)}/2
A second augmented voltage aVL is measured perpendicular to a line segment connecting the vertices RA and LL, with a length represented by a deviation of the measured LA voltage from the expected median value, (V(RA)+V(LL)/2), described as aVL=V(LA)−{V(RA)+V(LL)}/2. The atrial depolarization vector V(ad;depol) is the vector sum of the aVF vector and the aVL vector and is shown in FIG. 5 relative to a centroid CT of the triangle EQTR.
A P-Q time interval, normally of length Δt(p-q)≈120-200 msec, represents conduction time from initiation of atrial depolarization until initiation of ventricular depolarization, which is conventionally measured from the start of the P wave to the start of the Q swave.
Where the ICS is diseased or is affected by presence of Digitalis, the P-Q time interval may lengthen as the pulse rate decreases; a prolonged P-Q interval, substantially beyond 200 msec in length, is often evidence of atrio-ventricular block. An abnormally short P-Q interval, substantially below 120 msec in length, is often associated with hypertension and/or with paroxysms of tachycardia. The P-Q interval can also be shortened where the impulse originates in the AV node, or other atrial locations, rather than in the SA node.
The QRS time interval, normally of temporal length Δt(q-t)≈50-100 msec, represents conduction time from initiation of ventricular depolarization until initiation of ventricular repolarization, and includes spread of the electrical impulse through the ventricular muscle. The P signal is normally gently rounded, not pointed or notched, and has a temporal length ≈50-110 msec. One or more of the P, Q, R, S and/or T peaks may be inverted in FIG. 2, depending upon electrode placement. A QRS interval greater than about 120 msec in temporal length often indicates ventricular arrhythmia or a block of one of the bundles.
Normally, an S-T segment amplitude is approximately equal to a T-P segment amplitude. The amplitude of the S-T segment, relative to the baseline (e.g., elevated or depressed), and the shape of the T signal are often of interest. The T signal is normally rounded and slightly asymmetrical. Presence of a sharply pointed or grossly notched T signal may indicate presence of myocardial infarction (pointed segment) or of pericarditis (notched segment).
In some subjects, a beat (a single PQRST complex) is sometimes missed, as illustrated in FIG. 6. This arises from the particular physiology of that subject and has not (yet) been shown to arise unambiguously from the presence of high stress in that subject.
In normal ECG practice, ten or more electrodes including a ground electrode, are attached to the subject at selected, spaced apart locations. A chart of each PQRST complex is printed separately on a 1 mm×1 mm grid, with 1 mm (horizontal) representing 40 msec (0.04 sec time increment) and 1 mm (vertical) representing 0.1 milliVolts (mV amplitude). An upper limit on amplitude is usually 20-30 mm (2-3 mV). The chart is normally recorded at a velocity of 25 mm/sec or, alternatively, at 50 mm/sec. Measurement of an amplitude of 5 mm (0.5 mV) or less for all components in a PQRST complex is often seen in coronary disease, cardiac failure, emphysema and/or obesity. A T signal with unusually large peak amplitude (above 1 mV) may indicate presence of ischemia without infarction, or hyperkalemia, or psychosis.
ECG analysis is generally limited to medical diagnostics and to comparison of shifts with the passage of time of ECG parameters of interest. No substantial work has been done applying the ECG results to other areas of interest, such as authentication of an asserted identity of a candidate person, through analysis of selected ECG results to provide one or more physiologically based biometric indicia associated with the candidate person. Nor has any substantial use been made of evidence of a malady such as myocardial infarction or pericarditis as a characteristic for verifying the identity of a candidate person.
What is needed is a method and associated system for measuring one or more (preferably several) statistical parameters associated with PQRST complexes for a candidate person and authenticating, or declining to authenticate, the person's asserted identity with a reference person, using a comparison of the measured statistical parameter values (biometric indicia) with corresponding reference parameter values. Optionally, the comparisons should be cumulative so that the biometric indicia test can be made progressively more demanding, to minimize likelihood of commission of a type I error (positive result is false) and/or to balance the likelihoods of commission of a type I error and commission of a type II error (negative result is false) in these comparisons. These comparisons should also allow for possible changes with passage of time of PQRST complex characteristics for a candidate person. Preferably, evidence of presence of a malady in a reference person should be available for biometric use in comparison of a candidate person with this reference person.